Thanks to Google's indifference, the pirates can continue to sell ebooks no matter how many times copyright holders might complain. If Google takes a pirated ebook down in response to a DMCA notice, the pirates simply upload another copy of the same title.

If you pirates to stop distributing your works, you need to go after the pirates. Google is not your mommy. It won't kiss you and make everything all right.

DMCA requires platforms to take down very specifically identified infringements. The same work uploaded by another user is not necessarily an infringement, so another notification is required on behalf of the rights owner. Not policing for infringement is not a sign of indifference on the behalf of the platform. Policing is not required of the platform because it is impossible for it to do so.

Someone who can type into a word processor is to word processing as a script kiddie is to programming. I can type stuff into Word, but I couldn't use Word effectively without spending some time with it.

The difference is that committing a murder is illegal, but reporting that "escroc" is commonly associated with "Lyonnaise de Garantie" is not.
If someone said Lyonnaise de Garantie is a crook, and if that person was wrong, then Lyonnaise de Garantie should be going after that person.

Dropbox, as the service provider, does NOT have the right to say what is or is not copyrighted content

Of course they can. It's not libel, slander or otherwise illegal speech.

If you meant they don't have the right to take down content, that's not right either. The DMCA safe harbours make clear that a site isn't responsible for the actions of 3rd parties, but that doesn't prevent sites from policing their users if they want to, and it doesn't prevent sites from taking down any content they want to from their own sites.

What they can't do is send DMCA takedown requests for content to which they down own the rights. If they did this here, they are in the wrong. Unfortunately, very few face the consequences of sending false DMCA takedown requests.

It also goes on to say the if safe harbor were given to these sites, it would put too big a burden on networks to police their own material.

So what if it's a big burden for Viacom? That's Viacom's business problem, not Google's. It's just as laughable as hearing an entrepreneur say "it's too big of a burden to find costumers".

But let's say that "it's too big a burden" is a valid argument for a second. Viacom is known to intentionally hide the fact that its the one uploading its own material to Youtube. This has led to Viacom mistakenly sending takedown requests for material it itself placed on Youtube. If Viacom can't get it right with regards to its own material, it would be downright impossible for Google to get it right for Viacom's material, much less everyone's material. It would be an even bigger burden for Google, thus the status quo imposes the least burden.

Finally, what differentiates "these sites"? Their size? Their success? By that argument, Apple should be responsible for preventing bank robbers from using an iPhone to organize their crime. That's nonsense. Liability must lie with those who actually perform the illegal act.